In vitro shoot regeneration from transverse thin cell layers of apple leaves in response to various factors

Transverse thin cell layers (tTCLs) of apple (Malus X domestica Borkh.) leaves and conventional leaf segments of two apple cultivars ('Royal Gala' and 'Freedom') were used to investigate the effects of thidiazuron (TDZ), the position of the source leaves, and the period of regeneration, on the developmental outcome. The mean numbers of regenerated shoots per tTCL explant (4.4 and 2.1 for 'Royal Gala' and 'Freedom', respectively) and per conventional leaf segment (6.3 and 2.3 for 'Royal Gala' and 'Freedom', respectively) after 7 weeks of regeneration were equivalent to, or lower than the mean numbers of regenerated shoots (6.3 per tTCL, or 10.2 per conventional leaf segment explant) from the first leaves of 'Royal Gala' after 9 weeks of regeneration at the optimal level of TDZ. The numbers of regenerated shoots per tTCL explant of 'Freedom' did not increase from 7 to 9 weeks at the optimal level of TDZ (2.5 mg l(-1)). However, the number of shoots which could be regenerated from a single leaf using tTCLs was 6.5- to 17.5-fold higher than by using conventional leaf explants, depending on the genotype, the position of the source leaf, and the period of regeneration. A plant growth correction factor allowed growth and development to be compared using different parameters.